Processes for production of recombinant proteins using recombinant microorganisms have been used to obtain various heterologous proteins (Non Patent Literature 1). Various culture conditions have been studied to improve recombinant protein productivity. An effective way to improve productivity per unit time is increasing cell growth rate. Generally, the cell growth rate is known to increase as the culture temperature rises as long as the rise is within the range in which microorganisms can grow. At high culture temperatures, however, plasmids introduced into recombinant microorganisms for expression of heterologous proteins are less likely to be stably maintained in the cells and therefore more likely to be lost from the cells. Thus, the recombinant protein yield decreases even though the cells grow rapidly. At low culture temperatures, on the other hand, the plasmids can be stably maintained, but in this case the cell growth rate decreases so that productivity is reduced.
The demand for antibody drugs, among protein drugs produced using recombinant DNA techniques, has been rapidly increasing.
Antibody drugs are generally produced by affinity chromatography based on antibody-binding capacity. The most commonly used chromatography techniques are those that use carriers for antibody purification obtained by immobilizing a protein such as Protein A, Protein G, or Protein L on an appropriate resin. A particularly commonly used ligand having antibody-binding capacity is Protein A.
The affinity carriers using proteins as the ligands are required to have high quality as materials for drug production. Since the ligand proteins themselves are also required to have the same quality level as protein drugs and thus their production cost is high, the affinity carriers have not been available at low cost. The production cost of affinity carriers, which occupies a large portion of the production cost of antibody drugs, has been a major obstacle to reducing the production cost of antibody drugs. Therefore, there is a need for a method for preparing high-quality but inexpensive ligand proteins.
The inventors have already found a method for stably mass-producing a partial sequence of Protein A, which can cause Brevibacillus bacterial host cells to efficiently express and secrete large amounts of the partial sequence of Protein A, stably accumulate the protein in the culture fluid, and separate and collect it easily at high purity (Patent Literature 1).